This article is from the Bulgaria FAQ, by Dragomir R. Radev firstname.lastname@example.org with numerous contributions by others.
(by Graham Crowder)
There is a book/cassette combination in the Teach Yourself
Teach Yourself Bulgarian
A complete course for beginners
by Michael Holman and Mira Kovatcheva.
Publisher: Hodder and Stoughton.
This came out just a year or two back. There is an older
title "A Course in Modern Bulgarian parts 1/2" published
by Slavica Publishers, Inc (Columbus, Ohio).